Wound electrical capacitors are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. Radial lead capacitors are preferred in many applications over axial lead capacitors because of the space that is saved by their use and also because they may be directly inserted into a printed circuit board without bending of their leads. The problem of securing radial leads to the electrodes of wound capacitors with a strong, reliable bond was solved by the important contribution of Charles C. Rayburn in U.S. Pat. No. 3,040,415 and 3,134,059. The lead connection procedure of these Rayburn patents consists essentially of heating the electrical leads and forcing them, while heated, into dielectric layers, which extend outwardly at the edge of the capacitor so that the leads become embedded in the extended dielectric layers and are thereby firmly held in contact with the electrodes by melted dielectric after it is cooled.
The use of a single-sided metallized dielectric layer to form wound capacitors is highly desirable at the present time, in view of the rapidly increasing cost of supplies, especially the cost of conductive metals which make the use of the much thicker foil layers that have been used as alternatives to metallized dielectric layers relatively more expensive. The method of the Rayburn patents has been proposed for use in conjunction with alternating layers of single-sided metallized dielectric layers in which the dielectric layers extend from the edges of the capacitor in the opposite directions, as shown in FIG. 5 of the Rayburn Pat. No. 3,040,415; but the addition of alternating layers of heat-shrinkable material was not contemplated by these patents.
Another wound capacitor which used single-sided metallized layers is shown in Rayburn Pat. No. 3,364,401 in which an unmetallized margin is provided along one edge of each of the dielectric layers. However, the cost of providing the unmetallized margin on the dielectric material makes this type of single-sided metallized capacitor relatively expensive to produce.
OTHER TYPES OF CAPACITORS HAVE BEEN CONSTRUCTED USING METALLIZED DIELECTRIC LAYERS. For example, Great Britain Pat. No. 686,293 shows a capacitor which is made by using double-sided metallized layers that are separated by dielectric layers. The capacitor of this patent is wound so that the voids are substantially eliminated and the ends are sprayed with a metallizing end spray material. However, this capacitor requires two metallized surfaces rather than just one; and in addition, both of the metallized surfaces must be provided with a margin which makes the material costs relatively even more expensive than that of the previously mentioned Rayburn Pat. No. 3,364,401. Also, the dielectric material of the double-sided metallized layer does not contribute substantially to the value of the capacitor, and thus a relatively thick capacitor results for the amount of dielectric material that is used.
Electrical capacitors have also been constructed in which solid foil layers extend at opposite edges of the capacitor. An example of this type of a construction in which the foil layers are external beyond each other to a substantial extent is shown in Rayburn Pat. No. 3,267,343. Another example of the construction o a foil layer capacitor is shown in Bilsing et al Pat. No. 3,163,917. Although the foil layers of the Bilsing et al patent are separated by heat-shrinkable layers, the capacitor of the Bilsing et al patent utilizes axial leads, rather than radial leads and a separate insulating tube is used to secure the leads into the wound capacitor structure.
The capacitor of the present invention, by contrast, is formed by utilizing alternating layers of a heat-shrinkable dielectric material and a metallized nonheat-shrinkable dielectric material. The metallized nonheat-shrinkable dielectric layers extend beyond the heat-shrinkable dielectric layers at opposite edges of the capacitor and radial leads are secured to the edges of the capacitor in order to make contact with the metallized electrode layers. The leads are secured by heating them and forcing them into the edges of the capacitor where they melt the outer portions of the heat-shrinkable layers and preferably also the nonheat-shrinkable layers. Following the attachment of the leads to the capacitor body, a metallized end spray operation is performed which increases the retention strength of the leads and also reduces the resistance of the capacitor.
A capacitor constructed in accordance with the present invention has a number of decided advantages. These include:
1. Single-sided metallized dielectric layers may be employed in preference to foil electrode layers, thereby appreciably reducing the amount of metal and material costs required in the construction of the capacitor;
2. Single-sided metallized dielectric layers are employed which do not have margins, thereby further reducing the cost of constructing a wound capacitor;
3. Radial leads are provied which reduce the size of the capacitor and which allow for easy insertion of the capacitor in printed circuit board applications;
4. The radial leads of the capacitor of the present invention are securely mounted to the edges of the capacitor since they are held in place by melted dielectric material;
5. Increased end termination strength is provided by the increased gap spacing between the electrode layers since larger particles size end spray material may be employed;
6. The use of heat-shrinkable material as a separating layer makes it possible to provide for higher current capacities, improved dissipation factor, better control over the temperature vs. capacitance characteristic, improved corona threshold voltage and improved control over the final value of the wound capacitor;
7. No additional lead holding component is required to be included in the capacitor during construction; and
8. Voltage ratings may be adjusted with a large degree of flexibility due to the ease of adjusting the offset at the edge of the capacitor of the heat-shrinkable dielectric layers with respect to the metallized nonheat-shrinkable dielectric layers; and
9. Different combinations of dielectric materials can be used to change the characteristics of the capacitor so as to give a desirable temperature vs. capacitance curve or to improve corona threshold or dissipation factor, for example.